E. Migirdicyan

Spectroscopic effects of vibronic coupling between nearby 3nπ* and 3ππ* states of dimethylbenzaldehydes in a durene matrix

Abstract A vibrational analysis is presented of high resolution ππ* phosphorescence spectra of the 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1 h 1 and −1 d 1 , guests in durene host. As previously reported for the fully hydrogenated molecules, the energy gaps between the zero-point levels of the 3 ngp*, and 3 ππ* states of the guests deuterated on the aldehydic group are determined from the temperature dependence of their phosphorescence spectra and lifetimes. These vibronic energy gaps tend to be larger in the deuterated species.The existence of the vibronic coupling between 3 nπ* and 3 ππ* states of dimethylbenzaldehydes is suggested by the important activity, in the ππ* phosphorescence spectra, of nontotally symmetric modes involving the aldehydic group and the aromatic ring. Among these vibrations, the most active are the aldehydic CH and CD wagging modes which also show overtone activity. The intensities of the induced bands show a strong deuterium effect which in the 2,4 and 2,5 compounds is anomalous in that the deuterated species shows the higher induced intensity. The observation of phosphorescence spectra from each molecule in two different sites allows an assessment of the crystal field effect which is found to be moderately strong. These observations can be understood qualitatively on the basis of weak pseudo Jahn-Teller-like coupling between the 3 ππ* and 3 nπ* states which is subject to interference from Herzberg-Teller coupling involving at least one additional state. No convincing evidence is found for nonplanar distortions of the guest molecules.

Guest and host deuterium effects of the large ZFS parameters D of dimethylbenzaldehydes in durene single crystals

The ZFS parameters D of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h1 and -1d1 guests in perhydrogenated and perdeuterated durene single crystals are determined by comparing the experimental and calculated resonance curves. It is found that the deuterium substitution of the guest aldehydic group in a given host leads to the decrease of the D values and to the increase of the energy gaps ΔET between the zero-point levels of the 3nπ* and 3ππ* states of the guests. On the other hand, the perdeuteration of the host results in the decrease of ΔET with a corresponding increase of the D value of a given guest. The D value of 1 cm−1 determined for 2,5-dimethylbenzaldehyde-1h1 in perdeuterated durene is the lategest ever found for an aromatic carbonyl compound. Correlations between D and ΔET indicate that the ZFS parameters D of the guests are determined by contributions from both spin-spin and spin-orbit interactions between the 3nπ* and 3ππ* states. The large guest and host deuterium effects observed on the D values are attributed to the changes of the gaps ΔET of the guests.

On the electronic spectra of the duryl radical and the origin of the green emission of durene

Abstract The visible fluorescence, ultraviolet absorption and excitation spectra of the duryl (2,4,5-methylbenzyl) radical at 77°K have been observed after formation of the radical by photolysis of crystalline durene or rigid solutions. The transitions observed are analogous to those of the benzyl radical. The green emission of irradiated durene observed by Kallmann is attributed to the duryl first doublet-doublet fluorescence.

Laser-induced fluorescence spectra of phenylcarbene, 2-naphthylcarbene and 10,10-dimethylanthrylidene in glassy media at 77 K

Abstract The laser-induced fluorescence spectra of phenylcarbene (PC), 2-naphthylcarbene (2-NC), and 10,10-dimethylanthrylidene (DMA) were produced by excimer laser photolysis (KrF, 249 nm, 100 mJ) of the appropriate diazo compound in glassy 3-methylpentane at 77 K. The spectra cannot be obtained by laser flash photolysis at ambient temperature and, in the case of PC and DMA, cannot be obtained by conventional photolysis at 77 K.

Zero-field splitting of the ground and excited triplet states of 2-naphthylphenylcarbene studied by hole-burning on triplet-triplet fluorescence excitation spectra

Abstract Spectral holes were burned in the two main lines of the triplet-triplet 0-0 fluorescence excitation spectrum of 2-napththylphenylcarbene in n -hexane at 1.8 K. For the line assigned to pseudo-E/trans conformer, the central hole at the frequency of burning light has several narrow components separated by 0.3 ± 0.05 and 1.0 ± 0.1 GHz and a satellite doublet line on the low energy side, 14.5 ± 0.5 and 15.5 ± 0.5 GHz away. For the line assigned to the psuedo-Z/cis conformer, the central hole is accompanied by holes burned on the low energy side, 0.8 ± 0.1, 5.2 ± 0.1, 11.9 ± 0.1 and 16.2 ± 0.1 GHz away. The hole-burning pattern is explained by a model taking into account the zero-field splitting (ZFS) of the ground T 0 and excited T 1 triplet states as well as the selectivity of excitation relaxation by the intersystem crossing channel. Analysis provides information about the ZFS parameters of the T 1 state. For the pseudo-E/trans 2-NPC they are: D 0 = 0.47 ± 0.02 cm −1 , E 0 = 0.017 ± 0.003 cm −1 , D 1 = 0.038 ± 0.003 cm −1 and E 1 = 0.005 ± 0.001 cm −1 .

Temperature and deuterium effects on the phosphorescence lifetimes of dimethylbenzaldehydes in durene single crystals

Abstract The phosphorescence decays of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1 h 1 and −1 d 1 guests in perhydrogenated and perdeuterated durene single crystals have been studied between 4.2 and 120 K. The decays are exponential in the low (4.2 to ≈ 60 K) and high ( T > 90 K) temperature ranges and non-exponential in the intermediate range. The decrease in guest phosphorescence lifetimes σ observed between 15 and 60 K is essentially due to the thermal population of the nearby higher lying nπ* triplet state which decays with a much faster rate than the lowest ππ* triplet state. The significant increase in σ as temperature is reduced from 15 to 4.2 K can be accounted for by changes in the Boltzmann population of the spin sublevels of the lowest 3 ππ* state. While no host crystal deuterium effect is detectable on σ, the observed guest intramolecular deuterium effect is predominantly due to the reduction of radiationless decay rates from the lowest 3 ππ* states. One possible interpretation of the non-exponential phosphorescence decays in the intermediate temperature range is their attribution to the trans and cis conformers of the dimethylbenzaldehydes obtained by the rotation of the carbonyl group around the CC axis and present in thermally determined population in the durene host.

Hole-Burning Studies on Triplet-Triplet Fluorescence Excitation Spectrum of 2-Naphthylphenylcarbene

Abstract Spectral holes were burned on the triplet-triplet 0-0 fluorescence excitation lines of main conformers of 2-naphthylphenylcarbene in n-hexane and n-heptane at 1.8 K. The central holes, at a frequency of laser burning light have usually several accompanying satellite holes in a range of 1.5 GHz around and deep holes on the low energy side, 14.5–16.5 GHz away. Such a pattern of holes reflects the zero field splitting of the ground T0 and first excited T1 triplet states and the fact that an energy relaxation by the intersystem crossing channel is highly selective.

Zero-Field Splitting of The T0 and T1 States of 2-Naphthylphenylcarbene from Hole Burning Studies of Triplet-Triplet Transition

Abstract Spectral holes have been burned on the 0,0 fluorescence excitation (T0 → T1) line of 2-naphthylphenylcarbene in n-hexane at 1.8 K. A complicated spectrum of holes reflects zero-field splitting of both the triplet states, T0 and T1, and provides a way to estimate their splitting parameters D0, E0, D1 and E1. Comparison of the real hole burning spectrum with the simulated one suggests possibility of a distribution of burning yields and/or a distribution of homogeneous linewidths of 2-naphthylphenylcarbene molecules located in different sites of a Shpolskii matrix.

Site-selection spectroscopy of o- and m-xylylenes and their methylated derivatives in Shpolskii matrices

Abstract The vibronic structures of the site-selected fluorescence and excitation spectra of methylated o- and m-xylylenes in Shpolskii matrices at 5–10 K are analyzed. These biradicaloid species are photochemically generated from methylbenzenes.